This paper examined the requirements for frequency component or volume of warning sounds in the hearing experiments of sample warning sounds of Approaching Vehicle Audible System (AVAS) in an actual vehicle. The following facts were clarified. The high-frequency component in warning sounds cannot travel a long distance. The warning sounds should be measured from various directions and distances to the vehicle. Audible distance becomes longer when the difference in the sound pressure level between the warning signal and the ambient noise in any band is larger.